This invention relates generally to hot runner systems used to injection mold plastics, and more particularly to an improved heater cast used in multi-cavity hot runner edge gating.
This invention is an improvement over the "Hot Runner Heater" disclosed and claimed in the applicant's Canadian Pat. No. 976,314 which issued Oct. 21, 1975. While that patent discloses a heater having an integral electrical heating element, the present invention may be utilized with structure having a separate heating element. The applicant's prior patent referred to above discloses structure in which hot melt flows downwardly through a central injection passage to the bottom end of a nozzle member and then branches horizontally outward to the gate areas through four equally radially spaced convex portions. As stated in the introductory portion of this earlier patent, this structure was conceived to provide a multi-cavity system with improved heat transfer and heat loss characteristics which are, of course, very critical to operation of the system. However, this previous structure has the disadvantages that it is expensive to manufacture in that it must be milled, and that insufficient strength and cooling is provided by the relatively thin portion of the cavity plate directly between the heater nozzle member and the parting line when molding low height products. For instance, to mold closures or caps with a one-quarter inch high outer wall, the maximum thickness of the portion of cavity plate provided beneath the nozzle member would be only about one-eighth inch. This is unsatisfactory in that it eventually leads to failure of this thin portion of the cavity plate through repeated use or jamming of solidified material in the heater cast well. Furthermore, the flared shape of the heater nozzle member does not facilitate the provision of permanent stable insulation between it and the cavity plate to improve system performance in molding heat sensitive materials such as PVC, polycarbonate, polyester and acetal resins.